XHTML Resources:

w3schools.com
w3schools.com provides tutorial style descriptions of basic XHTML techniques and elements with examples.
Expertrating
Expertrating gives a step by step tutorial on how to do specific tasks in XHTML and also supplies examples.

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CSS Resources:

Silicon Bay Training
Silicon Bay Training shows a table with CSS properties and their values organized by the type of property.
CSS Values
CSS Values has an input box where you can type a CSS property and the page generates information about the property including possible value combinations and which browsers support it.
HTML Dog
HTML Dog is a guide to applying CSS, and the text is in pretty colors. It is good for beginners as it is almost like a best practices for incorporating CSS.

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Color or Design Issues:

Six Revisions
Six Revisions is a guide that shows color options, codes for colors, and explains transparency and color theory.
Browser News
Browser News provides solutions for design issues such as browser incompatibility, overcrowding, and bleeding edges.

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Copyright-free Images:

Morgue File
Morgue File allows you to search for free stock photos.
Getty Images
Getty Images allows you to search through categories for copyright- free images.